JPS6320056B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a circuit for detecting dropout (loss or reduction in signal amplitude) of a frequency modulated signal, and in particular, to a circuit that detects a dropout (loss or reduction in signal amplitude) of a frequency modulated signal, and in particular, a circuit that detects a dropout of a frequency modulated signal. The present invention relates to a dropout detection circuit having a hysteresis amount of . Such a circuit detects a dropout of a reproduced FM signal in a magnetic recording/reproducing device such as a video tape recorder, and then corrects the dropout.
It is used in circuits that compensate for signals delayed by 1H (one horizontal scanning period of television signals).

In a dropout detection circuit for a frequency-modulated signal, a detection level is set between the signal loss detection level and the signal recovery detection level in order to prevent circuit malfunctions due to signal vibration fluctuations or pulse noise in the dropout state. Difference is required.

FIG. 1 shows an example of a conventional circuit that attempts to realize the above-mentioned requirements, including an amplifier 2, a peak detection circuit 3,
It is configured to include a comparator 6, a reference level switching circuit 7, and a level conversion circuit 9. The input signal 1 is amplified by the first stage amplifier 2, and the peak value of the input signal 1 is detected by the peak detection circuit 3 and inputted to one input terminal 4 of the comparator 6. A reference level signal determined by a reference level switching circuit 7 is applied to the other input terminal 5 of the comparator 6. Dropout occurs in input signal 1, and peak detection circuit 3
The peak detection output level of the reference level switching circuit 7
When the signal dropout detection level is lower than the signal dropout detection level set in , the comparator 6 outputs a signal. Level converter 9 detects the signal output from comparator 6,
The signal loss detection level of the reference level switching circuit 7 is switched to the signal recovery detection level. The signal return detection level is set higher than the signal loss detection level.

Next, when the dropout signal returns and reaches the signal return detection level, the output of the comparator 6 is immediately cut off. The level converter 9 detects the output failure of the comparator 6 and thereby lowers the signal recovery detection level of the reference level switching circuit 7 to the signal loss detection level.

As described above, by switching the reference level to the comparator 6, a level difference is set between the signal loss detection level and the signal recovery detection level, and a hysteresis amount is provided between the two. At this time, in the configuration shown in Figure 1, even if the signal loss detection level changes,
The hysteresis width between the signal loss detection level and the signal recovery detection level is constant regardless of changes in the signal loss detection level. However, in order to prevent circuit malfunctions due to signal amplitude fluctuations or pulse noise in a dropout state, it is desirable to have a relative hysteresis width depending on the signal dropout detection level.

FIG. 2 shows an example of a conventional circuit that satisfies such requirements, including an automatic gain control circuit 11 and a peak detection circuit 1.
3. Consists of a reference level setting circuit 14 and a comparator 15. The input signal 10 is controlled by an automatic gain control circuit 11 so that it has a constant level at an output point 12. The peak detection circuit 13 detects the peak value of the input signal and inputs it to one input terminal of the comparator 15. A reference level signal determined by the reference level setting circuit 14 is applied to the other input terminal of the comparator 15. When a dropout occurs in the input signal 10 and it falls below the signal loss detection reference level of the comparator 15, the signal is output from the comparator 15 and the reference level setting circuit 1
The signal loss detection level of step 4 is switched to the signal recovery detection level. When the dropout signal returns and reaches the signal return detection level, comparator 1
The output of No. 5 is immediately cut off, and the signal recovery detection level is switched to the signal loss detection level. In the automatic gain control circuit 11, when the input signal level decreases, level correction is performed using a time constant of the automatic gain control circuit 11. Therefore, since the recovery time differs depending on the signal dropout level, it is possible to realize a relative hysteresis width depending on the signal dropout detection reference level. However, since automatic gain control circuits require a large capacity, they are not suitable for integration into integrated circuits.

An object of the present invention is to provide a dropout detection circuit that has a relative hysteresis width corresponding to a signal dropout detection level and is suitable for integration.

According to the present invention, the output of an amplifier having gain switching means is connected to one input terminal of a comparator via a peak detection circuit, and a variable level setting circuit is connected to the other input terminal of the comparator. , the dropout detection circuit is characterized in that the output terminal of the comparator is connected to a level conversion circuit, and the gain of the amplifier is switched based on the output signal of the level conversion circuit.

Next, the present invention will be explained in more detail with reference to the drawings.

FIG. 3 shows a dropout detection circuit according to one embodiment of the present invention. In FIG. 3, an output terminal 26 of an amplifier 18 having gain switching means is connected to one input terminal 27 of a comparator 23 via a peak detection circuit 21, and the other input terminal 28 of the comparator 23 is connected to a reference A level setting circuit 24 is connected, an output terminal 25 of the comparator 23 is connected to a level conversion circuit 22, and an output terminal of the level conversion circuit 22 is connected to a gain switching control terminal 29 of the amplifier 18.

The input signal 17 is input to the amplifier 18, but in a steady state, it is amplified by the amplifier (A) 19 with a fixed gain, and the input signal 1 is input to the peak detection circuit 21.
A peak value of 7 is detected. This output is comparator 2
The signal is input to one input terminal 27 of 3. Comparator 2
The other input terminal 28 of No. 3 is set to a constant reference level by the reference level setting circuit 24, and is compared with the peak value of the input signal. Input signal 1
When a dropout occurs at 7 and the peak value level falls below the reference level, a signal is output from the comparator 23. Level conversion circuit 22 detects this output signal and switches the gain of amplifier 18. As a result, the amplifier 18 is switched from the amplifier (A) 19 having a fixed gain to the amplifier (B) 20 having a different fixed gain, thereby lowering the gain. The gain of amplifier (B) 20 is set lower than that of amplifier (A) 19. Therefore, after detecting a dropout, the signal in the dropout state is amplified with a lower gain than in the steady state, so a level difference caused by the difference in gain is obtained in order to return to the reference level, which is equal to the signal dropout detection level. A hysteresis width is obtained between the signal recovery detection level and the signal recovery detection level.

When the dropout signal returns to the reference level, the comparator 23 cuts off the output signal. The level conversion circuit 24 detects this output signal interruption and switches the gain of the amplifier 18 to the initial gain (amplifier (A) 19) in the steady state.

FIG. 4 is a signal waveform diagram in FIG. 3.
For the sake of simplicity, a case will now be described in which a signal 30 whose amplitude is modulated by a triangular wave is input. This input signal 30 is input to the amplifier 18, detected, and compared with a reference level by the comparator 23. However, when the detected output of the input signal 30 decreases below this level, the gain of the amplifier 18 is lowered. Output terminal 27 of peak detection circuit 21
An output waveform 31 is obtained. The output signal of the comparator 23 at this time is indicated by 32.

By adopting such a configuration, a hysteresis width is obtained between the signal loss detection level and the signal recovery detection level, and the hysteresis width is determined by the switching gain difference of the first stage amplifier, so the detection reference level can be When it is varied, a signal recovery detection level having a constant amount of hysteresis determined by the switching gain difference of the amplifier 18 according to the detection reference level can be obtained.

Next, a specific circuit configuration of the present invention will be explained using FIG. The input signal is applied to the common base 3 of the differentially connected transistors 37 and 38 and the differentially connected transistors 45 and 46, respectively.
It is input between 3 and 34. The collector of the transistor 38 has a differentially connected transistor 35,
Thirty-six common emitters are connected. Similarly, the common emitters of differentially connected transistors 43 and 44 are connected to the collector of the transistor 46. At the same time transistors 35, 36, 4
3 and 44 are double differentially connected. The bases of the transistors 36 and 43 are connected in common and connected to the reference bias power supply 61, and the bases of the transistors 36 and 43 are connected in common.
The collectors of the transistors are commonly connected to the common load 42 and the base of the transistor 50. Common load 4
2 and the transistors 37, 35, 45, 4
The collector of No. 3 is connected to the power supply terminal 54. The emitter of the transistor 50 is connected to a peak detection capacitor 52 and a resistor 53, and the other ends of the capacitor 52 and resistor 53 are grounded. Transistors 54 and 55 of conductivity type opposite to the transistor 50
are differentially connected, each collector has a load 5
6 and 57, the base of the transistor 54 is connected to the emitter of the transistor 50, and the base of the transistor 55 is connected to the reference level setting terminal 59.
It is connected to the. The common base of the transistors 35 and 44 constituting the bi-differential circuit is the transistor 5.
The collector of the transistor 54 is connected to the output terminal 58, and outputs a dropout detection signal. One side is connected to the power supply,
A variable resistor 60, the other end of which is grounded, is connected to a reference level setting terminal 59 to set a dropout detection reference level.

In the differential circuit constituted by transistors 54 and 55, the input signal level is compared with a dropout detection reference level, and in a steady state, the input signal level is higher than the reference level, so transistor 54 is in a cut-off state. It is in. Therefore, no current is supplied to the resistive load 56 and no signal is output to the output terminal 58. At this time, since the transistor 55 is in an active state, a voltage is generated across the load resistor 57, and the transistor 3
A bias potential is supplied to the bases of 5 and 44. The voltage generated across the load resistor 57 is set higher than the reference bias power supply 61. Therefore, the transistors 36 and 43 constituting the bidifferential circuit are in the disconnected state, and the transistors 35 and 44 are in the active state, and the input signal is transmitted from the differential transistors 45 and 46, the emitter resistors 47 and 48, and the load 42. The signal is amplified by an amplifier A, and its output level is detected at a peak detection terminal 61 and inputted to the base of a transistor 54 constituting a differential circuit.

On the other hand, when a dropout occurs in the input signal and the input signal level falls below the dropout detection reference level, the transistor 54 switches to the active state, a voltage is generated across the load 56, and a signal is output from the output terminal 58. . At this time, the transistor 55 switches to a disconnected state, and the load 5
No current is supplied to transistors 35 and 4.
The base bias voltage of 4 is then cut off. Therefore, the transistors 35 and 4 forming the double differential circuit
4 is switched to the disconnected state, while 36,4
3 is switched to the active state. Therefore, the input signal is amplified by amplifier B comprising differential transistors 37 and 38, emitter resistors 39 and 40, and load 42.

The emitter resistors 39 and 40 are the emitter resistors 47,
Since the gain of amplifier B is set to be larger than 48, the gain of amplifier B is lower than that of amplifier A, and a level difference occurs due to the difference in the gains of amplifiers A and B until the signal in the dropout state recovers and reaches the reference level. , a hysteresis width determined by the gain difference between the amplifiers A and B is obtained between the input signal dropout detection level and the input signal recovery detection level.

When the signal in the dropout state returns and reaches the reference level, the transistor 54 is turned off again, the output signal of the output terminal 58 is turned off, and the transistors 36 and 43 are turned off. Amplifier B is switched to amplifier A and the steady state gain is switched.

In the configuration described above, the amount of hysteresis is set by the gain difference between the two amplifiers, so when the reference level is varied, the input signal loss detection level and input signal return detection level are adjusted for each reference level. Since it can be set to a constant ratio and has a differential circuit configuration, it is suitable for implementing this dropout detection circuit into a semiconductor integrated circuit.

It should be noted that the present invention is not limited to the above-mentioned embodiment, and in particular, the circuit configuration shown in FIG. 5 can be modified as appropriate within the scope of the present invention.

[Brief explanation of the drawing]

FIG. 1 is a simple block diagram illustrating a conventional example of a dropout detection circuit, FIG. 2 is a simple block diagram illustrating another conventional example of a dropout detection circuit, and FIG. 3 is a dropout detection circuit using the present invention. FIG. 4 is a diagram showing signal waveforms from a dropout detection circuit using the present invention, and FIG. 5 is a circuit diagram embodying the present invention in a circuit configuration. 1, 10, 17...Input signal, 30...Input signal waveform, 31...Peak detection circuit output waveform, 32
...Output signal waveform, 35-38, 43-46, 5
0,54,55...transistor, 39,40,
42, 47, 48, 56, 57, 53...resistance,
41, 49, 53...constant current source, 52...capacitor, 60...variable resistor, 61...bias power supply.

Claims (1)

[Claims] 1. An amplifier having a gain switching function, a detector for detecting the output of the amplifier, a comparator for comparing the output of the detector with a reference level, and means for switching the gain of the amplifier based on the output of the comparator. A dropout detection circuit comprising: